Fix a bug with taxonomy rendering.

- In `layouts/_default/taxonomy.html`, the `.Data` result does not
  provide the same information that `layouts/_default/terms.html` does
  for being able to identify the plural value of the term.
- This change adds `.Data.Singular` and `.Data.Plural` to provide
  similar capabilities.
- This *may* be incompatible with templates that check for `{{ if ne
  $taxonomy "Pages" }}` if the `page.Params` has either the singular or
  plural values as keys.

hugolib/site.go

@@ -843,6 +843,8 @@ func (s *Site) newTaxonomyNode(t taxRenderInfo) (*Node, string) {
 		n.Date = t.pages[0].Page.Date
 	}
 	n.Data[t.singular] = t.pages
+	n.Data["Singular"] = t.singular
+	n.Data["Plural"] = t.plural
 	n.Data["Pages"] = t.pages.Pages()
 	return n, base
 }